Transport in Plants

• Explain the need for transport systems in multicellular plants in terms of size and surface-area-to-volume ratio.

• Describe the distribution of xylem and phloem tissues in roots, stems and leaves of dicotyledonous plants.

Transport systems and surface area to volume ratio

• Single celled organisms have large surface area: vol ratio and can obtain all their requirements (oxygen, CO2, water and minerals) by diffusion. The sugar they make is available throughout the cell.

• Large plants need to transport water and minerals up from the roots. Sugars need to be moved from the leaves where they are made to other areas of the plant. In order to do this they need transport systems

Tissues that carry out movement of water, minerals and sugars

• A Tissue is group of similar cells that carry out a particular function

• Xylem tissue is made of Xylem vessels, fibres and parenchyma cells. Xylem transports water and minerals.

• Phloem tissue is made of Sieve tubes and companion cells. Phloem transports sugars.

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Distribution of vascular tissue in plant roots

• Xylem forms a cross shape in the centre with areas of phloem between the arms

• The vascular (transport) tissue is surrounded by a layer of endodermis

• The Pericycle is a layer of meristem (dividing) cells inside the endodermis

Distribution of vascular bundles in stem

• In the stem the vascular bundles of Xylem, Cambium and Phloem are arranged around the edge of the stem

• Xylem is always on the inside of the bundle,

• Cambium is a layer of meristem cells in the middle of the bundle

• Phloem is always on the outside of the bundle

Distribution of vascular tissue in leaves

• In leaves the vascular tissue is seen as the midrib and veins in the leaf

• The xylem is always closer to the top surface of the leaf

• The phloem is below the xylem

Describe the structure and function of xylem vessels, sieve tube elements and companion cells.

Xylem tissue

Phloem (longitudinal section)

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Week 10

Explain, in terms of water potential, the movement of water between plant cells, and between plant

cells and their environment.

• Pure water has a water potential of zero

• Any solute dissolved in water lowers the water potential and makes it more negative

• Water always moves from an area of higher water potential to an area of lower water potential

Movement of water into plant cells• This cell has a water potential of

-500kPa

• It is bathed in pure water with a water potential of 0kPa

• So water enters the cell down the water potential gradient (from higher to lower) by osmosis

• Eventually the pressure exerted by the water entering the cell equals the pressure exerted by the cell wall on the contents, water stops going into the cell

• The cell becomes turgid

A pair of adjacent cells, A and B, have water potentials of -1000kPa and 1200kPa respectively. Which cell will gain water from the other

Movement of water out of plant cells• If a plant cell is put into a

solution with a much lower water potential than the cell then water will leave the cell by osmosis.

• The vacuole shrinks and then the cytoplasm shrinks and becomes smaller in volume

• Eventually the cell membrane pulls away from the cell wall

• The cell becomes plasmolysed.

What is in the space between the cell wall and the plasma membrane of the plasmolysed cell?

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Movement of water between plant cells

• If adjacent cells have different water potentials then water will move between them, by osmosis, down the water potential gradient

Complete the question on water potential

Movement of water through plants

• A, the Apoplast pathway. Water and dissolved ions move through the cell walls between the cellulose molecules

• B, the Symplast pathway. Water goes into the cell through the plasma membrane into the cytoplasm. It moves from cell to cell through the plasmodesmata

• C, the Vacuolar pathway. Water goes into the cell through the plasma membrane into the cytoplasm and then into the vacuole.

• Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata.

• Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream.

© Pearson Education Ltd 2008This document may have been altered from the original

• Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to:

the Casparian strip,

apoplast pathway

symplast pathway,

xylem and stomata.

Water movement into the plant• Water goes into the root cells and

moves across the cortex by osmosis, water can move by any of the pathways

• At the Casparian strip in the Endodermis water is forced out of the Apoplast pathway and into symplast or vacuolar pathway

• Water and ions pass through proteins in the plasma membrane into the cytoplasm

• Nitrate ions are actively pumped from the endodermis cells into the xylem

• This lowers the water potential in the xylem so water follows by osmosis

Movement up the stem and out of the leaves

• Pumping ions into the xylem forces water to follow by osmosis

• Water can rise up stems about 3 metres by this process

• Water evaporates from leaves through the stomata, water moves through the leaf by osmosis down the water potential gradient

• Water leaves the xylem creating tension in the xylem, this is why the xylem needs to be strengthened with lignin, to prevent collapse.

• Cohesion between water molecules means that the whole column of water is pushed upwards from below and pulled upwards from above

Loss of water from leaf by Transpiration

• Osmosis moves water from xylem to palisade and spongey mesophyll

• Water evaporates from the mesophyll cells into the intercellular spaces

• Water diffuses from intercellular spaces out through stomata

Factors that increase transpiration rates

• Number of leaves

• Number of stomata

• Cuticle

• Light

• Temperature

• Relative humidity

• Air movement/ wind

• Water available

• More leaves = larger surface area

• More stomata = more spaces for evaporation

• More cuticle= slower evaporation

• Stomata open in sunlight

• Higher temp = faster evaporation, faster diffusion through stomata

• Lower gradient slows water loss

• Maintains water potential gradient

• Little water in plant closes stomata and reduces water loss

Animation link

• http://www.kscience.co.uk/animations/transpiration.swf